1. Field of the Invention
This invention is related to integrated circuit phase shifters, and, more particularly, to an integrated circuit configuration, which can provide a controlled continuously variable phase shift.
2. Description of the Prior Art
Continuously variable phase shifters using varactor diodes have been employed in the prior art. A typical configuration uses these devices as the terminating element of the two output ports of a quadrature hybrid circuit. By adjusting the capacitance of such phase shifters, the phase of the reflected signal is changed, thereby producing the desired phase shift. Varactor diodes have a sufficiently broad capacitance range to produce variable phase shifts of up to 180 degrees. However, these devices are not readily amenable to monolithic microwave integrated circuit (MMIC) processing, and therefore, they do not present the MMIC circuit designer with a practical phase shifter configuration.
Prior art attempts to provide MMIC compatible phase shifting including the use of MESFET's which are MMIC compatible. For such devices, it has been demonstrated that by varying the gate-source capacitance of a FET a zero degree to fifteen degree variable phase shift can be obtained. These approaches have used MESFETs in a common source amplifying mode, which have the objection of being unidirectional devices.
Another prior art approach to providing a continuously variable phase shifter employs a dual gate FET used as both a phase shifting and amplifying element. In this configuration with a radio frequency (rf) voltage source having an impedance connected to a first gate and a load connected to the drain, the transmission phase can be controlled by varying the bias voltage at the second gate, which changes the parallel resonance between the inductor and the source to gate capacitance. This circuit has been demonstrated to provide a 0 degree to 90 degree phase shift. However, this approach has the above-mentioned limitation of operating only in a common source amplifying mode and is unidirectional.
A further description of a related but subsequently invented phase shifter is disclosed in U.S. application Ser. No. 438,028, filed Nov. 1, 1982, now U.S. Pat. No. 4,471,330, issued Sept. 11, 1984, entitled "Digital Phase Bit for Microwave Operation" of R. J. Naster et al, and assigned to the Assignee of the present application. In that application, a digital phase shift is achieved using a pair of field effect transistors (FETs), operated in an on-off mode, each FET employing a shunting transmission line, and each shunted combination being serially connected by a transmission line whose length is selected to cancel reflections.